The present invention relates to a fan unit designed for use in duct systems, as for example in ventilation and air conditioning systems, and has a freely running radial impeller without a spiral casing and has a driving motor for the impeller. Such fans may be installed in ducting or the like of round or rectangular cross-section and have the advantages and useful effects of an axial fan--the motion of the air or other gas is straight through the fan and there is a simple system for installing the fan in position. In addition, such fans provide the useful effects of a radial fan by providing a high pressure and generally quiet operation. For these reasons, they are used frequently and for a great number of different purposes. Installing this type of fan in position is about as simple as attaching a duct or pipe in position. In this type of fan, the inlet air flows in the same direction as the outlet air, that is, they both flow generally axially.
However, known designs of fans of this type have the shortcoming that the air comes out of the impeller with a high degree of spin and, thus, the path of the volumetric flow of air in the duct downstream from the fan is helical. This spin is responsible for substantial energy losses. A substantial part of the kinetic energy supplied by the impeller to the flow is not usable so that such fans have a comparatively low efficiency. One of the purposes of the present invention is to overcome this shortcoming.
Stated somewhat differently, a purpose of the invention provide a fan unit of the type referred to above and having the advantages of the previous designs but not having the disadvantages of the previous designs. This purpose is accomplished by designing the fan to provide at its outlet side a generally parallel flow. The flow of air in the duct downstream from the fan impeller does not have any spin and, therefor, the energy losses referred to earlier are avoided.